General Description

Is the neutron round? If not, so what?

It turns out that the "roundness" of the neutron charge distribution
(measured by the electric dipole moment) is connected at a deep level to a
rather fundamental question - "Why is there any matter in the Universe?"
Matter and anti-matter were created in equal amounts during the Big Bang. Most
(all but one part in a billion) was annihilated in subsequent matter/anti-matter collisions.
Clearly (and thankfully) a small excess of matter survived to form our Universe.
Theories that purport to explain this invariably predict that the neutron is not
quite perfectly round.

The
neutron electric dipole moment was first measured in 1950 by
Smith, Purcell, and Ramsey
at the
Oak Ridge Reactor - the first intense neutron source.
This first measurement showed that the neutron was very nearly round
(to better than one part in a million). In the last sixty years the precision
of the measurement has improved by more than six orders of magnitude; the
neutron is known to be round to better than one part in a trillion!

Detailed Description

The statistical sensitivity (sigma) of an EDM experiment depends on the electric field (E),
the number of neutrons (N) and the neutron storage time (tau):

sigma ~ 1 / (E * sqrt(N*tau))

The nEDM experiment at the Spallation Neutron Source was conceived by
Golub and Lamoreaux in a seminal paper.
This approach takes advantage of an amazing confluence of superfluid helium
properties to substantially increase E, N, and tau: